<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <canvas id="canvas" style="border: 1px solid #aaa; display: block; margin: 50px auto;">
        当前浏览器不支持Canvas，请更换浏览器
    </canvas>
    <script>
        window.onload = function () {
            var canvas = document.getElementById("canvas");

            canvas.width = 800;
            canvas.height = 800;

            var context = canvas.getContext("2d");

            context.lineWidth = 10;
            context.lineJoin = 'miter'; //默认效果
            // context.lineJoin = 'bevel'; //衔接效果
            // context.lineJoin = 'round'; //圆角效果
            // context.miterLimit = 20;    //当使用lineJoin是miter时
            drawStar(context, 20, 300, 400, 400, 30);
        }

        function drawStar(cxt, r, R, x, y, rot) {
            cxt.beginPath();
            for (var i = 0; i < 5; i++){
                cxt.lineTo(Math.cos((18 + i*72 - rot)/180*Math.PI)*R + x, -Math.sin((18 + i*72 - rot)/180*Math.PI)*R + y);
                cxt.lineTo(Math.cos((54 + i*72 - rot)/180*Math.PI)*r + x, -Math.sin((54 + i*72 - rot)/180*Math.PI)*r + y);
            }
            cxt.closePath();
            cxt.stroke();
        }
    </script>
</body>
</html>